When two-color printing is effected in a multiprinter where each printing wheel has two groups of type letters one of which is provided with red ink and the other is provided with black ink, a system adapted to invert the rotating direction of the printing wheel according to a selected color can decrease the rotation amount of the printing wheel and hence reduce the printing time rather than a system adapted to rotate the printing wheel in a single direction.
To change the rotating direction of the printing wheel, one proposal is to control the rotating direction of a motor which is the driving source of the printing wheel, and another proposal is to interpose a swing gear between the drive source motor and the printing wheel and selectively engage the swing gear with another gear.
The former proposal, however, is practically unable to rotate only the printing wheels among others in opposite directions because the same motor also drives a hammer, paper feeding roller, etc. in addition to the printing wheels.
The latter proposal is liable to produce errors in stop positions of the printing wheels because a loss time where no power is transmitted is inevitable upon engagement or disengagement of the swing gear, and this unables a reliable mechanical control of placement of the printing wheels according to the rotation amount of the motor.